Disk drive devices using various kinds of media, such as an optical disk, a magneto-optical disk, and a flexible magnetic disk, have been known in the art. In particular, a hard disk drive (HDD) has been widely used as a storage device of a computer and has been one of indispensable disk drive devices for current computer systems. Moreover, the HDD has found widespread application such as a removable memory used in a moving image recording/reproducing apparatus, a car navigation system, a cellular phone, or a digital camera, as well as the computer, due to its outstanding characteristics.
A magnetic disk used in the HDD has a plurality of concentric data tracks. On the respective data tracks, a plurality of servo data containing address information and a plurality of data sectors including user data are recorded. A plurality of data sectors are recorded between the respective servo data. A head element portion of a head slider supported by an actuator pivoted by a voice coil motor (VCM) can read and write data to and from the data sector by accessing a desired data sector according to the address information of the servo data.
When the magnetic disk is not rotating, the actuator and the head slider is retracted to a retract position. As a scheme for retracting the head slider, ramp loading/unloading scheme and contact start stop (CSS) scheme are known in the art. During moving between the retract position and over the magnetic disk, the HDD controls pivoting of the actuator by velocity control of the VCM. Since the VCM velocity is proportional to a back electromotive force (EMF) voltage, measuring the back EMF voltage of the VCM enables the velocity control of the VCM. Specifically, the back EMF voltage detection circuit detects the back EMF voltage of the VCM. The controller controls to supply VCM current to the VCM so that the detected back EMF voltage becomes a target value. Typically, the back EMF voltage detection circuit detects the back EMF voltage by detecting a value expressed by a linear function of the back EMF voltage.
When the resistance of the VCM coil varies in accordance with temperature changes, the relationship between the VCM current and the back EMF voltage is changed. Therefore, the back EMF voltage detection circuit compensates changes of the resistance of the VCM coil by changing circuit parameters. However, as a discrete value is applied to the circuit parameter, the changes of the resistance of the VCM coil are sometimes not compensated completely. Also, detection errors occur due to the circuit configuration of the back EMF voltage detection circuit. Hence, the detection value of the back EMF voltage detection circuit contains an offset proportional to the VCM current supplied to the VCM.
Hereafter, the above is specifically explained. FIG. 9 is a circuit diagram showing an example of circuit configurations of the back EMF voltage detection circuit 221. The back EMF voltage detection circuit 221 includes operational amplifiers OP1 and OP2, balance resistors R1, R2, R3a, R3b, R4a, and R4b. The resistors R3a and R3b have the same resistance R3, and the resistors R4a and R4b have the same resistance R4. The resistor R2 is a variable resistor and its resistance R2 is the above-described circuit parameter. Further, a sense resistor Rs is connected serially to the VCM coil.
The back EMF voltage β satisfies the following equation:β=Vm−Rm×Im.  (1)wherein Vm, Rm, and Im represent a voltage of the VCM coil (VCM voltage), a resistance of the VCM coil (VCM resistance), and current running through the VCM coil (VCM current) respectively. It is assumed that the VCM current Im is constant and invariable. The back EMF voltage detection circuit 221 can detect β by detecting Vm and Im directly or indirectly.
The operational amplifier OP1 and the resistors Rm, Rs, R1, and R2 constitute a bridge circuit in their entirety and the resistance R1 and R2 determine the gain at the first stage of amplifier. Specifically, the R1/R2 is the gain of the first stage of amplifier. When this bridge circuit is balanced, the output is equal to the VCM voltage Vm. When the bridge circuit is balanced, the following equation is satisfied:R2/R1=Rm/Rs.  (2)
The operational amplifier OP2 and the resistors R3a-R4b are a differential amplifier for outputting the output of the operational amplifier OP1 as a back EMF voltage (or multiplied back EMF voltage by a predetermined number). The resistances R3 and R4 determine the gain of the output stage of the amplifier. Specifically, R3/R4 corresponds to the gain of the output stage of amplifier. The output Vout of the operational amplifier OP2 is given by the following formula:R4/R3×(Im×Rs×R2/R1−Vm)+Vref.  (3)
The output Vout of this operational amplifier OP2 and the back EMF voltage of the VCM 15 has a proportional relationship.
Here, in order to establish the relationship of the formula (3), it is necessary that the bridge circuit is balanced, that is, the relationship of the formula (2) is satisfied. If this relationship is not satisfied, an error occurs between the output Vout of the operational amplifier OP2 and the back EMF voltage β of the VCM 15, and as a result, the output value (detected value) of the back EMF voltage detection circuit 221 does not reflect the actual back EMF voltage. The VCM resistance Rm varies according to temperatures. Therefore, if the resistances of the resistors R2 and R1 are constant, the formula (2) becomes unsatisfied due to the temperature changes. Therefore, the back EMF voltage detection circuit 221 calibrates the resistance R2 of the resistor R2 to compensate the variation of the VCM resistance Rm. However, since the resistance R2 is a digital value, the formula (2) sometimes is not satisfied. Also, operational errors of hardware in the back EMF voltage detection circuit occur.
FIG. 10 is a graph schematically showing the relationship between the A/D conversion value AD of the detected value of the back EMF voltage detection circuit 221 and the VCM current value Im (digital value) supplied to the VCM by a command of the controller in a state that the VCM has stopped. Since the VCM has stopped, the detected value AD normally should be 0 regardless of the value of the VCM current Im. However, the detected value AD varies according to the VCM current Im due to the above-described detection errors of the back EMF voltage detection circuit 221. Specifically, the detected value AD is represented by a linear function with the gradient α representing the varying rate of the detected value AD and the offset v0 of when the VCM current Im is 0.
Thus, the detected value of the back EMF voltage detection circuit 221 contains errors variable according to the VCM current. Therefore, the controller performs a correction operation of the detected value to compensate the errors to control the VCM using the result of the correction operation. To carry out the correction operation, the controller is required to obtain a correct value of the gradient α. The velocity control by a correction process using an incorrect a may cause an oscillation of the actuator. However, as understood from the above explanation, this gradient α varies depending on temperatures and operational conditions of the HDD. Therefore, the controller conducts a calibration to obtain a more correct value of the gradient α as disclosed in Japanese Patent Publication No. 2005-174539 (“Patent Document 1”).
The calibration of the gradient α is carried out before the head slider moves from a retract position to above the magnetic disk or before the head slider moves from above the magnetic disk to the retract position. The gradient α can be identified by the calibration with applying different current to the VCM in a state that the actuator has stopped and obtaining the back EMF voltage at that time. For example, in a state that the actuator is being pushed against a crash stop at the inner peripheral side or the outer peripheral side, two different VCM current values are applied to the VCM 15. The gradient α can be identified from the relationship between the respective current values and the detected values thereto.
However, if the head slider is positioned above the magnetic disk, a loud noise arises when the actuator is pushed against a crash stop. Or, performance degradation is supposed to occur due to the process time to push the actuator against the crash stop. The Patent Document 1 discloses a calibration by performing seek of the actuator between the two stop positions, i.e., a present position and a target position. This enables to conduct the calibration without pushing the actuator against the crash stop.
However, servo control utilizing servo data read by the head slider is required for performing the seek. The calibration of the gradient α is a calibration for the VCM control utilizing a VCM back EMF voltage so that the calibration without the servo control is preferred. Besides, when the servo control is unstable, noises by the VCM current for the servo control increase. Therefore, calibration without servo control is preferred for an accurate calibration. In another aspect, it is preferred that the calibration is preformed to control the VCM accurately using the back EMF voltage even when the servo control cannot be preformed.